If you mean taking a tax deduction on the 1040 long form, that we the taxpayer don’t know yet. The IRS will announce the minimum alternate tax for 2012 that one has to pay if they have a large amount of deductions.

So, yeah, the rule used to be: any loss NOT covered and NOT paid for by insurance.

this would be claimed on Sch A under casualties, and you will need to know the market value of the car when the accident occurred, you will reduce that by the amount the insurance paid etc
it is entirely possible that it might not even qualify as a deduction even if you were able to use the Sch A
you use that form if you can itemize greater than your standard deduction

How did you get the gap insurance? If you bought it yourself independent of the car loan, you are fine; if you got the gap insurance as part of the loan, you WILL get a 1099-C for cancelled debt income which *is* taxable on your return.

Your loss is limited to what the car was worth, so you won’t have a casualty loss at all.